Crowding out is the process in biology where the introduction of new species to an environment causes new competition with previously existing species, usually while radically changing the environment.

When a new species is introduced to an environment, one of two things typically happens. One possibility is that the new species is unprepared for the new environment and dies out quickly. There would be a strong evolutionary pressure to adapt quickly or become extinct.

The second possibility is that the previously existing species are unprepared for the new one. The new species easily and quickly reproduces. As a result of the increase, the competing species may become extinct. Those competing species are said to be crowded out.